This invention relates to injection molding metal and ceramic powders, commonly known as Powder Injection Molding (PIM) or Metal Injection Molding (MIM). Conventional PIM processes are of two types. In the first, a carefully selected system of thermoplastic resins and plasticizers are mixed in an amount to fill the void volume of the powder. Such mixing operations are carried out in a high shear mixer, and at a temperature sufficient to decrease the viscosity of the plastics and uniformly mix the powder and resins. The resultant product is pelletized. The pellets are then reheated and injected into a cooled die where the thermoplastic resins increase in viscosity to a point where the part can be ejected from the die. Some of the binder is then removed. This is accomplished using a variety of techniques including solvent extraction, wicking, sublimation, and decomposition. This fraction of the binder is removed to provide sufficient porosity to the part and so that the remaining binder can decompose thermally and be removed from the part. This latter step is done at a low enough temperature to preclude substantial reaction of the binder with the metal powder. The above-noted techniques are well known in the art and are disclosed for example, in U.S. Pat. Nos. 4,404,166 (wicking), and 4,225,345 (decomposition). All require substantial processing time and specialized apparatus in order to first mix, and then remove the binders.
The second type of PIM process utilizes a plastic medium consisting of an organic binder and modifiers dissolved in a solvent. After mixing the binder with solvent, metal powder, and modifiers, the plasticized mass is injected, under pressure, into a heated mold. Water is expelled from the organic binder, under heat, causing an increase in viscosity sufficient to support the part during ejection from the die. Further heating of the part increases its strength and volatilizes the solvent, leaving sufficient porosity so the remaining binder can be volatilized and substantially removed at a low enough temperature that the powder does not coalesce.
Both types of processes require that additional processing be performed on the parts between the molding and sintering steps, in order to open the body of the part or to remove certain or all of the binders or byproducts. This increases equipment costs, processing time, and overhead as well as making the process more difficult to control.
In both processes, temperature control is critical for proper mixing, rheology, and part strength. This also necessitates additional equipment cost and process controls. For example, in the former processes, the solidified powder/binder mixtures need to be re-melted prior to forming on an injection molding press. This increases equipment cost due to the added complexity of presses and related tooling needed to inject the mixtures, as well as the cost of high intensity, thermally controlled mixing apparatus. In the latter described process, the need for proper temperature and the mix viscosity work opposite to each other; the screw required to inject the high viscosity mix produces heat that must be removed in order to keep the mix cool.
In any PIM process it is desirable (if less than 97% of theoretical density is acceptable for the finished part) to substitute a percentage of more expensive fine powder for a coarser powder which may be only a tenth the cost. This substitution decreases the amount of shrinkage taking place during sintering, and leads to better dimensional stability. With the above PIM process, however, the increased pre-sintered density, that naturally occurs when mixing powders of dissimilar sizes, further increases the viscosity of the mixture, compounding process control and overhead problems.
Because of these drawbacks, these processes are seldom economical for part runs of less than 5000 pieces. Even with larger quantities, the inability to use prototyping and short run molding techniques (such as silicone rubber tooling) increases preproduction and engineering costs.